Ammonium Formate as a Safe, Energy-Dense Electrochemical Fuel Ionic Liquid

While solid and liquid energy carriers are advantageous due to their high energy density, many do not meet the efficiency requirements to outperform hydrogen. In this work, we investigate ammonium formate as an energy carrier. It can be produced economically via a simple reaction of ammonia and formic acid, and it is safe to transport and store because it is solid under ambient conditions. We demonstrate an electrochemical cell that decomposes ammonium formate at 105 °C, where it is an ionic liquid. Here, hydrogen evolves at the cathode and formate oxidizes at the anode, both with ca. 100% Faradaic efficiency. Under the operating conditions, ammonia evaporates before it can oxidize; a second, modular device such as an ammonia fuel cell or combustion engine is necessary for complete oxidation. Overall, this system represents an alternative class of electrochemical fuel ionic liquids where the electrolyte is majority fuel, and it results in a modular release of hydrogen with potentially zero net-carbon emissions.


Electrochemical experiment preparation Cell preparation
Ammonium formate is dried in a beaker at 100°C under an ammonia atmosphere for at least 4 hours (in practice, the beaker was covered in parafilm, keeping the parafilm away from the heat, and anhydrous ammonia gas was flowed at 50 SCCM into the beaker). Less than 30 g of ammonium formate was dried at a time to ensure sufficient drying. The dried ammonium formate was then stored in a desiccator and used within a few days.

S4
The custom H cell used in these experiments was designed to look like Figure 1b. Namely, the H cell had a glass frit separator and each compartment had two additional openings on the side of gas inlet and outlet. Before each experiment, the outside of the glass H Cell was wiped down with hexanes to remove the silicone oil. The inside of the cell was then thoroughly rinsed with MilliQ water, followed by 10% nitric acid, followed by MilliQ water, followed by acetone, followed by MilliQ water. Then, the cell was dried at 80 °C to remove all water.
After removing the cell from the drying oven, each compartment of the glass cell was filled with 5.4 g of dried ammonium formate. 0.6 g of MillQ water was then added to each compartment. The main opening at the top of the cell compartments were parafilmed closed, but the gas ports were left open for air flow. The cell was then placed in an oil bath at 105°C for 30 minutes to melt the ammonium formate-water mixture. The electrodes were then inserted and the electrochemical experiments conducted. The electrode area in the electrolyte was approximately 1.5±0.1 cm 2 .

Electrode preparation
For metal foil cathodes, the foils were polished using 1500 grit sand paper for >1 minutes, sonicated for at least 2 minutes in DI water, and then dried at 80 °C to remove all water. This was done before every experiment unless specified otherwise. Due to the thinness of Pt foils, these foils were not polished with sand paper. Instead, they were dipped in 10% nitric acid for at least a minute (a few hours if a new Pt foil) and then sonicated in DI water and dried as described above.

S5
The electrodes themselves were attached to a small aluminum current collector with Kapton tape, and the aluminum current collector was attached to an aluminum rod with aluminum tape. The potentiostat was then connected to the aluminum rods via alligator clips. Note that contact resistance can be a problem, and lots of tape and finger-tight applied pressure is useful for assuring low resistance between the potentiostat and the electrode foil ( Figure S1). The electrode area in the electrolyte was approximately 1.5±0.1 cm 2 . Figure S1. Example electrode preparation for gold electrode. First, the electrode is taped with Kapton tape to an aluminum current collector (A,B). A strip of aluminum foil is often used to help improve electrical contact as well as to create a "tab" (A, B) to help remove the tape after the experiments (a so-called "babybel®" tab). Then, aluminum conductive tape is used to attach S6 the current collector to an aluminum rod to which the alligator clips will attach (C,D). Additional Kapton tape is applied as necessary to guarantee good electrical contact.

Electrochemical experiments
All electrochemical experiments were performed using a Biologic VMP3 potentiostat. A typical experiment proceeds as follows: 1) A PEIS test to determine the initial impedance, followed by a manual resistance compensation at 85% 2) An LSV at 10 mV/s from 0 V to 0.5 V 3) An open circuit rest for two minutes to add a stir bar and let equilibrate 4) A subsequent PEIS to record any changes in the resistance 5) A chronopotentiometry step with 10 mA applied current for 60 minutes 6) A final PEIS step to note any changes in resistance during the previous step

Product Quantification
Hydrogen quantification was done using an SRI gas chromatograph (GC). A thermal conductivity detector with nitrogen gas as the carrier was used to detect hydrogen gas. Reaction conditions are the same as in other experiments, namely dried ammonium formate combined with 10 w/w% water melted at 105°C. Nitrogen gas was flowed at 5 standard cubic centimeters per minute (SCCM) into the headspace of each compartment. For quantification of hydrogen, a post trap of 0.1 M aqueous boric acid (H3BO3) was used to catch any ammonia before the gas flow S7 entered the GC. The GC was calibrated in the range of interest to quantify hydrogen gas partial currents and Faradaic efficiencies ( Figure S3). For quantification of carbon dioxide, quantification via GC was not possible due to the necessity of a post trap for the ammonia. The CO2 would dissolve in the post trap, leading to inaccurate GC readings. Instead, a calcium hydroxide trap was used. An empty 20 mL vial was used to collect any possible liquids (sometimes small quantities of electrolyte can travel over) that may be carried along with the gas. The outlet gas from this empty vial was bubbled through a CO2 trap.
The trap is prepared by dissolving calcium hydroxide in water. When CO2 is bubbled, it reacts to form calcium carbonate (an insoluble solid) which can later be separated and weighed to calculate the amount of CO2 captured. The experiments performed to quantify CO2 included a chronopotentiometry run at 7.1 mA for 1 hour. Through trivial mole balance calculations, it is expected to form 13.24 mg of calcium carbonate and requiring roughly 9.8 mg of calcium hydroxide. At room temperature the solubility of calcium hydroxide in water is 1.6 mg/ml, therefore for a 10 mL trap the maximum dissolved amount is 16 mg. This posed a risk as the concentration of dissolved calcium hydroxide would be reduce as experiment progressed, possibly making the trap less effective. Therefore, a total of 32 mg (double saturation amount) of calcium hydroxide was added to 10 mL of water to prepare the trap.
The extraction of calcium carbonate from the trap must be performed with great caution as the expected yield is small and losses must be minimized. Additionally, the excess calcium hydroxide must be carefully washed off else it will contaminate the final extracted product and overestimate the amount of CO2 in the trap. The process for extraction is detailed as follows: S8 1. Transfer the contents of the trap to a 15 mL Falcon tube 2. Centrifuge at 6000 rpm for 10 minutes and remove the water -It was better to leave around 2 mL of water and use a pipette to prevent loss of solids.
3. Add 10 mL of fresh water to the trap container to wash it for any residual and transfer it over to the falcon tube. Sonicate to disperse the solids for efficient washing of the calcium hydroxide.

Repeat
Step 3 two more times to ensure all calcium hydroxide is washed away.

5.
Transfer the contents into a smaller centrifuge tube (1.5 mL), centrifuge at 15,500 rpm for 5 minutes and discard the water. Multiple washes of the larger falcon tube are recommended to prevent loss of any product. 6. After, water is discarded we are expected to have a wet solid. The final drying was performed in an oven at 80 °C overnight. An aluminium foil was used to cover the lid of the centrifuge tube. Several holes were made to allow water to leave.

Solid was removed from centrifuge tube and weighed
Ammonia was quantified using the indophenol method as described in previous work. 1

Differential Scanning Calorimeter
Samples of ammonium formate were dried at 80 °C under an ammonia atmosphere for > 6 hours before being sealed by parafilm and stored in a dessicator until tested on the Differential Scanning Calorimeter (DSC). A TA Instruments DSC250 DSC was used to calculate the melting temperature and enthalpy of fusion for ammonium formate. The DSC was calibrated using a S9 sample of benzoic acid under the same conditions. The samples were placed in a sealed boat and then the temperature was ramped at a speed of 1 °C per minute from ambient condition to 140°C. Integration of the heat flux to calculate enthalpy was done using the trapezoidal method on the raw data for a given window centered on the heat flux peak ( Figure S5). The exact window bounds had little impact on the integrated enthalpy or melting temperature (< 5% change in enthalpy and negligible change in temperature).

COSMO-RS Calculations
COSMO-RS is a program that can be used for calculating thermodynamic properties of (mixed) fluids. 2 It is a combination of COSMO with a statistical thermodynamics treatment of interacting surfaces making it more powerful. 3 It has been used successfully to model solute-solvent systems and shown to perform well for multi-phase solvents over temperature regions. 4,5 In this case, we have a ternary solvent of ammonium, formate, and water as ammonium formate is largely present in the dissociated form while water primarily exists as one molecule. We are interested in ammonia vapor pressures hence it is declared as the solute. For the species in this system, we were able to use COSMO-RS libraries. COSMO-RS outputs activity of ammonia in the solvent along with the Gibbs free energy of solvation. These values can be used to compute the equilibrium vapor pressure above the solvent using the equation shown below, is the total pressure, is the activity coefficient of species , , is the solvation free energy of species , is the gas constant, and is the temperature.
Comparison to Thermochemical Decomposition

Influence of Temperature on Electrochemical Kinetics
Assume an anodic reaction obeys Marcus kinetics: (1) where n is the number of electrons transferred (we will assume the first electron transfer is the RDS with n−1 transferred after the RDS), F is Faraday's constant, is the constant prefactor, λa is the reorganization energy, and ηa = ϕa−ϕa,eq > 0 is the overpotential relative to the equilbrium potential for the anodic half-reaction. R and T are the ideal gas constant and temperature, respectively, and cM is the molar concentration of the reactant. We can quickly see that: (3) We can see that both the expression for current as well as the Tafel slope ( ) will approach the Butler-Volmer values in the limit η << λ. We will assume η < λ for the rest of this derivation (lowoverpotential regime).
We can then take some temperature derivatives: So we see that in all cases, at constant overpotential, increasing temperature will lead to increasing currents.
But what happens to the overpotential in the case of increasing temperature? S14 (8) From the above, we see that anodic overpotentials decrease as temperature increases (by our original assumptions, − > 0). The cathodic version of the above is straightforward with a simple sign switch (ηc < 0): Similarly, we see that cathodic overpotentials decrease with increasing temperature (remember that ηc < 0). We can then subtract the anodic derivative from the cathodic derivative to get an expression for the change in total cell potential with temperature (note that i = ic = ia): S15 (17) We know that − ∆ Let's assume we have non-gas reactants turning into gaseous products, ∆ > 0. Therefore, all terms in the above equation are positive and we find: For an electrolyzer, ∆ϕ < 0, so this means that the overall cell potential (|∆ϕ|) required to drive a reaction at some constant current, i, will decrease as temperature increases. For a fuel cell, ∆ϕ > 0, so this means that the overall cell potential (|∆ϕ|) we extract from a reaction at some constant current, i, will increase as temperature increases.
Supporting Analysis S16

Energy Density Calculations
As mentioned in the main text, there are multiple ways to calculate and compare the energy density of different molecules. One standard method for calculating energy density is to calculate the higher heating value (HHV), which is generally defined as the heat of combustion where the products are cooled to 25°C. The lower heating value (LHV) has multiple definitions, but generally does not include the latent heat of vaporization for water (e.g., the products of combustion are kept at 150°C). For the equilibrium potential, we care about the Gibbs free energy of the combustion reaction, i.e., the useful work that can be extracted from the reaction. We define combustion in this case as the reaction of one mole of fuel with stoichiometric oxygen to produce a combination of water, nitrogen gas, or carbon dioxide. The raw data and resulting energy densities on a volume basis are provided for select fuels (Table   S1). As can be seen, the calculations accurately predict the absolute entropy of ionic solids to within 10% error.

Resistance of cell over time
As discussed in the main text (Figure 2), the solution resistance changes throughout the course of the chronopotentiometry hold (see Experimental Procedure above). This change is likely due to changing electrolyte composition over time, such as an increase in acidity at the anode.
During the course of the one hour chronopotentiometry test, the solution resistance increases by ca. 3-5 Ω, regardless of anode material ( Figure S2). At 10 mA applied current, this corresponds to an increased voltage of ca. 30-50 mV during the course of an hour, which matches with the observer slight increase in cell voltage ( Figure 2). S19 Figure S2. Solution resistance before and after the chronopotentiometry test. PEIS was done before and after the 60 minutes of 10 mA applied current, and the solution resistance was extracted from the PEIS by finding either where the imaginary part of the impedance crosses zero or the value of the impedance at a frequency of 10 kHz. Each point represents the average of at least three trials.

Hydrogen quantification
First, the GC was calibrated for hydrogen gas ( Figure S3).
Solution Resistance ( ) S20 Figure S3. Calibration curve for hydrogen quantification on GC The experimental setup is described above. Using a Pd anode and Pt cathode, the following Faradaic efficiency data was collected ( Figure S4). The Faradaic efficiency reported in the main text ( Figure 3) was an average of the data points collected after one hour had passed to allow for the system to reach steady state.
S21 Figure S4. Faradaic efficiency toward hydrogen at the cathode over time. The system reached steady state after ~60 minutes due to the large residence time of the cell and the post-traps.

Carbon dioxide quantification
Carbon dioxide quantification was performed as described above. Two trials were taken with the following raw data results. These values were averaged and reported in the main text ( Figure 3).

Thermodynamic landscape
The thermodynamic landscape presented in the main text (Figure 4), was calculated as follows.
Each step of the landscape was calculated relative to the final products, which are nitrogen, water, and carbon dioxide at 25°C. The "energy" at each step, is therefore simply the negative of the reaction enthalpy (or Gibbs free energy) of the reactants at each step reacting to form the products at the final step. The change is sign is for intuitive interpretation of the plot. Note that an assumption in this equation is that Δ (25 ∘ ) = Δ (120 ∘ ) = Δ and that even for the Gibb's free energies at 120°C, the values are still relative to the products at 25°C, not the products at 120°C (the products at 120°C are, in fact, 38.5 kJ/mol less stable than the products S23 at 25°C). The raw values for the thermodynamic landscape ( Figure 4 in main text) are provided (Table S5).

Differential Scanning Calorimeter
The enthalpy and temperature of fusion for ammonium formate were calculated as described above. The DSC heat flux was calibrated using benzoic acid, which was known to have a melting temperature near that suspected of ammonium formate.
S25 Figure S5. Example plot of DSC data for ammonium formate. The peak was integrated to find the enthalpy and the weighted average temperature was used to find the peak temperature.

Temperature Dependence Referenced Experiments
Experiments in a three-electrode setup were performed at the regular conditions (105°C, 90 w/w% ammonium formate and 10 w/w% water), as well as at room temperature (saturated solution of ammonium formate in water, assumed to be approximately 22°C) and at 80°C with S27 75 w/w% ammonium formate and 25 w/w% water (chosen to be close, but under the saturation point at that temperature). In all cases, the same experimental setup as previously described was used (Pd anode and Pt cathode) with the addition of a Pt wire as a pseudoreference. In addition to anodic overpotentials versus a Pt pseudo-reference, the systems at 22°C and 80°C were calibrated with a ferricyanide redox couple so that the results could be plotted vs SHE and vs the formate oxidation potential (Table S8). Unfortunately, ferricyanide is not stable at 105°C, so exact overpotentials vs the equilibrium potential could not be determined at that temperature.

Temperature Dependence on System Equilibrium Composition
This system is a unique combination of water, ammonia, and formic acid. As a function of temperature, we expect to see water in liquid and gaseous form, formic acid in liquid, gaseous S28 and anion form, and ammonia as a gas and a cation. Accordingly, we can generate a variety of phase diagrams of the ammonium formate condensed phase ( Figure S6) and the vapor phase ( Figure S7). Supporting references